The invention relates to an optical measuring apparatus comprising an optical sensor which is connected to an optical coupler via an optical delay line and an optical path. The coupler is connected to an electro-optical transducer unit by a connecting fibre. The transducer unit supplies and receives light pulses and which supplies voltage pulses whose amplitudes correspond to the intensities of the received light pulses to a measuring circuit via at least one electrical line. The measuring circuit determines the physical quantity detected by the optical sensor from said amplitudes and transfers the result to a display.
An optical measuring apparatus of the type defined in the opening paragraph is disclosed in U.S. Pat. No. 4,293,118. The apparatus includes an optical pressure sensor, in which the freely movable parallel fibre ends of an optical delay line and of an optical path formed by an optical fibre terminate. A mirror is arranged opposite the fibre end faces, both the fibre end faces and the mirror surface being provided with opaque layers having identical grating structures. The movable fibre ends are mechanically connected to an elastic diaphragm, so that a pressure-dependent excursion of the diaphragm causes to move and hence the facing grating-structure layers are moved relative to each other, as a result of which the amount of light which is reflected back into the fibre ends by the mirror varies as a function of the pressure. The pressure exerted on the diaphragm can thus be determined by measuring the light intensity. The light is reflected back into the same fibre ends from which it issues, so that the light issuing from the fibre end of the optical delay line is also reflected back into the optical delay line and on account of the greater pathlength is subjected to a higher attenuation than the light propagating via the shorter optical path. Moreover, mechanical loads, variations in ambient temperature and changes in the transmission characteristics of the optical delay line as a result of aging result in additional attenuation-dependent measurement errors. Compensation for such errors requires the use of an intricate correction circuit which should allow for the instantaneous degree of attenuation of the light pulses passed through the optical delay line.